1. Field of the Invention
This invention relates to a control valve operable to distribute hydrodynamic pumped fluid between a plurality of fluid paths under the influence of an applied external hydrostatic load and is particularly applicable in an agricultural tractor and trailer combination to the distribution of hydrodynamic fluid between a trailer braking circuit, an implements circuit and a fluid reservoir under the influence of the tractor hydrostatic braking system.
2. Description of the Prior Art
A system of this general kind is already known, for example, from French Pat. No. 1450557 in which a single spool valve distributes hydrodynamic pressure fluid between a trailer braking circuit, an implements circuit and a fluid reservoir under the influence of a pilot slave cylinder incorporated in the hydrostatic braking circuit of a tractor. In this prior system, the pump is required to work under load continuously, even when the required degree of trailer braking has been achieved. This can lead to overheating of the hydraulic fluid and also results in unnecessary pump wear.
In another prior proposal, illustrated in BRITISH PATENT SPECIFICATION No. GB 2115894A the aforesaid problems are overcome by arranging for the trailer braking circuit to be isolated once the desired degree of trailer braking is obtained, enabling the pump loading to be reduced under such conditions. However, this arrangement employs a flow controlled spool valve to control the distribution of pumped fluid between implements and the trailer brakes, while using another spool valve to control the flow of fluid to a reservoir in response to an input signal from a hydrostatic slave cylinder. The functions required by this arrangement give rise to a complicated and consequently expensive mechanism.